Your previously normal horse has developed lameness, which you notice as a stiff, short-strided gait that is worse on the turns and worse on hard ground. Giving your horse phenylbutazone (Bute) dramatically improves the signs of lameness for a period of time, but the lameness persists and gets slightly worse. With these clinical signs, the fear of many horse owners is that the horse is developing navicular disease, and that the horse's career might be finished. Obtaining a very accurate diagnosis is very important in these cases, as numerous conditions or problems other than navicular disease can lead to similar clinical signs.
Historically, navicular disease has referred to a chronic forelimb lameness resulting from pain originating in or near the navicular bone, with horses often having abnormal radiographic signs involving the navicular bone. With the dramatic changes that have taken place in the imaging of bones and soft tissues (scintigraphy, ultrasonography, computed tomography or CT, and magnetic resonance imaging or MRI), we have learned that a wide variety of tissues other than the navicular bone can be involved in creating pain in the heel region of the horse's foot.
Anatomically, the navicular bone acts as a pulley to redirect the direction of the forces that are applied through the deep digital flexor tendon (DDFT) as it courses to attach onto the distal phalanx (coffin bone) within the foot. The navicular bone is suspended in this position by supporting ligaments, both on its proximal (upper) and distal (lower) surfaces. Between the DDFT and the navicular bone lies the navicular bursa, a small fluid-filled sac that ensures good gliding motion between these two structures during locomotion. In some horses with pain originating from the heel region of the foot, only one of these structures might be involved. In other horses, many of these tissues can be involved. The diagnostic workup is structured to help determine which of th